1. Field of the Invention
This invention relates generally to apparatus and methods for treating fluids which contain hydrogen sulfide and, more particularly, to apparatus and methods for treating geothermal steam which contains significant amounts of hydrogen sulfide prior to disposal of the steam.
2. Background Discussion
Large subterranean reservoirs of naturally occurring steam and/or hot aqueous liquids (geothermal fluids) are found at many locations throughout the world. These geothermal reservoirs are especially prevalent in regions where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole, and/or geyser activity. As an example, geothermal fluid reservoirs are fairly common along the rim of the Pacific Ocean, a region long known for its frequent volcanic eruptions.
Collectively, and in some cases individually, such geothermal reservoirs represent a vast, generally self-renewing, reserve of thermal energy which can often compete economically with crude oil when heat is needed or can be used. Consequently, in addition to more traditional therapeutic uses, geothermal fluids, especially geothermal steam and water (brine), have been used as sources of heat for: heating buildings, industrial processes, providing domestic hot water, and other heating purposes. As an example, domestic heating provided by geothermal energy accounts for about thirty percent of the net energy consumed in Iceland.
Although efforts continue to be directed toward expanding the direct use of geothermal fluids for heating purposes, such heating is site-restrictive. Furthermore, factories and other users of geothermal heat cannot always be located near geothermal sources, which are often in remote locations.
Accordingly, greater efforts, particularly in the past several decades, have been directed toward the increased use of geothermal fluids for the production of electric power. Advantages of using geothermal steam in this manner are that electric power has a broader range of uses than heat, and electric power can usually be distributed over wide areas using existing power networks so that its use is not site-restrictive. From about 1940 to about the mid 1970's, worldwide geothermal power generating capacity increased at an average annual rate of about 7 percent. However, since about the mid-1970's, in response to steep increases in the cost of crude oil and in response to crude oil shortages during that period, the annual worldwide growth rate of geothermal electric power generating capacity has been about 19 percent.
As an example of the importance of geothermal energy in this country, geothermal steam-operated power plants at The Geysers in Northern California reportedly produced about 7.4 million megawatts of electric power in 1986, enough to satisfy about 2 percent of the electric power needs of the entire State of California. Significantly, the use of geothermal steam to produce this amount of electric power at The Geysers represents a displacement of about 11.1 million barrels of crude oil valued at about 200 million dollars at 1986 crude oil prices. More generally, in about 1980, the United States Geological Survey estimated that the geothermal resources of this country were equivalent to about 430.times.10.sup.9 barrels of oil, not counting the energy which, in the future, may be derived from rock of low porosity and from magma.
Geothermal steam (such as is used at The Geysers) is the easiest, most efficient, and least costly of the aqueous geothermal fluids to use for electric power generation. As an illustration, geothermal steam, at a typical wellhead pressure of between about 140 psig and about 150 psig and at a typical wellhead temperature of between about 320.degree. F. and about 370.degree. F., can be used in the manner of boiler-generated steam in steam-turbine generators. Geothermal water or brine, in contrast, is more difficult to use for power generation, requiring either the converting of some of the geothermal liquid into steam or the use of the geothermal liquid to vaporize a secondary liquid in a closed cycle, binary system. In addition, many serious production and handling problems are caused by the extremely corrosive nature and scale-forming characteristics of most geothermal brines. Although efforts are being made to overcome these problems, the use of geothermal brine for the production of electric power is still usually much more costly than the use of geothermal steam for that purpose.
It might be assumed, because geothermal steam is abundant in some locations and is at a high natural temperature and pressure, that costs associated with the use of geothermal steam for power production are minimal. Such is not the situation, however. Ground leases and/or mineral rights must be procured, and geothermal steam-production wells must then be drilled to steam-producing formations, which, for example, at The Geysers, are typically at depths of about 6,000 feet. The average cost of drilling each such production well is reportedly about 1.32 million dollars. Since, as a rule, about 20,000 pounds/hour of geothermal steam are needed to produce one megawatt/hour of electricity at The Geysers, a typical 110 megawatt power plant in that location requires about 2.2 million pounds of steam per hour to operate at full capacity. About 20 production wells, at a capital expenditure of about 26 million dollars, are usually required to provide this amount of steam on a sustained basis. Substantial additional capital expenditures are required for steam-conducting pipe and associated steam-handling equipment, most of which must be in relatively large sizes in order to conduct the requisite large flow of steam, often over distances as great as a mile, from the production wells to where the electric power is generated.
Other additional costs associated with the use of geothermal steam for electric power generation, as opposed to the use of boiler-generated steam, relate to treatment of the steam for contaminants naturally contained in the steam as it is produced from the ground. In this regard, geothermal steam typically entrains significant amounts of well and formation debris which ordinarily must be separated from the steam before the steam can be used in power-generating turbines. Moreover, geothermal steam normally also contains appreciable amounts of non-condensable gases, such as hydrogen sulfide, ammonia, and carbon dioxide. Of these gases, hydrogen sulfide, which may be present in concentrations as high as about 500 parts per million (ppm) by weight, currently causes significant emission control problems in geothermal steam production and use because emission standards in many localities now strictly regulate the amount of hydrogen sulfide which may be discharged into the atmosphere.
Prior to the recent introduction of strict hydrogen sulfide emission standards, geothermal steam, which is commonly produced and used in relatively remote locations in this country, seldom required treatment for hydrogen sulfide emissions. Now, however, to comply with recent strict emission standards, substantial amounts of hydrogen sulfide generally must be removed before the ultimate disposal of geothermal steam or its condensate. The equipment required for such hydrogen sulfide removal or treatment is expensive to purchase and operate, and these related costs add to the overall cost of producing electric power by the use of geothermal steam.
One hydrogen sulfide emission problem, particularly relevant to the present invention, relates to the disposal of "surplus" geothermal steam which, for the purpose describing the present invention, is defined as the minimum amount of steam which normally must be produced in order to maintain the production wells in good operating condition and to keep the pipelines hot and ready for immediate steam delivery but which, for various reasons, cannot be used for the generation of electric power. Typically, this minimum production amount of steam is equal to between about 15 percent and about 30 percent of full steam production. Obviously, when (as is usual) surplus steam contains hydrogen sulfide, the abatement of hydrogen sulfide is an important consideration in determining how to dispose of the steam.
Related, at least indirectly, to the hydrogen sulfide abatement problem associated with the disposal of surplus steam is the manner in which geothermal steam power plants are owned and operated. Such power plants are most commonly divided into a steam-producing facility and a power-generating facility, each of which is separately owned and operated. Power-generating facilities are, for example, commonly owned and operated by utility companies whose traditional business has been electric power generation, and steam-generating facilities are commonly owned and operated by oil and gas companies whose traditional business has been the extraction of liquid fuels from the earth. Due to such separate ownership and operation, neither of the two facilities comprising the power plant is directly concerned with problems of the other facility, and even operational problems which affect both facilities cannot always be solved as practically or as efficiently as might be possible if both facilities were commonly owned and operated.
With regard to hydrogen sulfide abatement, the power-generating facility ordinarily has responsibility for controlling hydrogen sulfide emissions from geothermal steam which it accepts and uses. However, whenever the power-generating facility temporarily stops accepting steam from the steam-producing facility because of a power-generation malfunction, the steam-producing facility has the responsibility for meeting hydrogen sulfide emission standards for whatever amount of geothermal steam (that is, surplus steam) it continues to produce while the power-generating facility is "off-line." Although prior to recent impositions of strict hydrogen sulfide emission standards in most localities, the producing facility could usually discharge surplus steam into the atmosphere without treatment, extensive and costly apparatus for removing hydrogen sulfide from the steam are now needed even though the surplus steam may be produced only infrequently, and often for only short intervals of time.
As is more particularly described below, power-generating facilities now generally control hydrogen sulfide emissions by treating the non-condensable gases (including hydrogen sulfide) which are separated from steam condensate after power generation. Such steam condensation and non-condensable gas separation are now typically performed in relatively small-capacity condensers already installed downstream of the steam turbines. Prior to the imposition of strict hydrogen sulfide emission standards, the specific function of these condensers was to increase steam turbine efficiency by condensing the low-energy steam discharged from the turbines, thereby creating a vacuum at the turbine outlets to increase turbine efficiency. The recently added, non-condensable gas treatment equipment may consist of known types of hydrogen sulfide abatement apparatus, more specifically mentioned hereinbelow.
However, hydrogen sulfide abatement systems useful in power-generating facilities for the full-time treatment of low-energy, turbine exhaust steam are generally not economically practical for the treatment of intermittent flows of high-energy, surplus steam in steam-producing facilities. The same is true for other processes, including the chemical treatment of hydrogen sulfide using caustics and hydrogen peroxide. Furthermore, the economics of using such known hydrogen sulfide abatement systems or processes cannot be improved by constructing systems which could be transported from one steam-producing facility to another--even if it were possible to construct such transportable systems--because the power generating problems which result in the production of surplus steam tend to be random in nature, and immediate treatment of surplus steam produced by the steam-producing facilities is necessary to comply with emission standards.
For these and other reasons, geothermal steam-producing facilities presently have an important need for effective, yet economical, apparatus and methods for treating intermittent, high flows of surplus geothermal steam so as to enable disposal of the steam in compliance with existing hydrogen sulfide emission standards. Accordingly, it is a principal objective of the present invention to provide such a hydrogen sulfide treatment apparatus and method.